There are different ways of joining railway rails together to form tracks. One such way is to bolt rails together to form jointed track. In this form of track, lengths of rail, usually around 20 meters in length, are laid and fixed into position. In the UK the track lengths are traditionally fixed to sleepers, and in the USA they are traditionally fixed with crossties, or simply ties. Once laid, the track lengths are then joined together with steel plates, known as fishplates or joint bars.
Small gaps are deliberately left between the rails, which are known as expansion joints, to allow for thermal expansion of the rails in hot weather. Additionally, the holes through which the fishplate bolts pass are normally oval to allow for expansion.
Unless well maintained, jointed track provides a characteristic bumpy, noisy and uncomfortable ride due to the presence of the expansion joints, and is unsuitable for high speed trains because it is too weak.
The rail industry commonly uses Continuously Welded Rail (CWR) on all major tracks. In this form of track, the rails are welded together for several kilometers, to form one long continuous rail. This avoids the need for expansion gaps, and because there are few joints the rail is very strong and provides a smooth surface for high speed running. Because of its strength, trains traveling on welded track can travel at higher speeds and with less friction. Welded rails are more expensive to lay than jointed tracks, but are significantly cheaper to maintain.
As mentioned above, rails expand in hot weather (and contract in cold weather). As welded track has very few expansion joints, it could become distorted in hot weather and cause a derailment. In order to compensate for thermal expansion in the welded rail, it is laid with significant tension. This process is commonly known as stressing, and ensures that the rail will not expand much further in subsequent hot weather.
The load applied to the rail to produce the tension is calculated so that, at a locally determined temperature, the rail will expand to reduce the tension to zero. This temperature is known as the stress free temperature (SFT). The SFT varies from country to country, and in the UK is normally 27° C.
FIG. 1A of the accompanying drawings is a diagram which illustrates a method for tensioning lengths of rail.
Two lengths of rail 10 and 20 are laid upon a number of sleepers. The rail lengths are laid such that a calculated gap 30 exists between the cut ends. The gap is calculated based on the SFT, and the expansion coefficient of the rail. Each length of rail 10, 20 shown in FIG. 1A is 900 meters long, however each length of rail can be of any length. The end 40, 50 of each length of rail 10, 20 furthest from the gap 30, is clipped onto the sleepers for a length of more than 20 m. This is known as the anchor length. Tensing machines (normally hydraulic) are attached to the free ends of the rails 60, 70 and the rails are pulled towards each other with a force of approximately 60 tons. This force can vary depending on the type of rail and individual site conditions. This tensile force extends the rail lengths 10, 20 until the free ends 60, 70 meet. Once the free ends meet, they may be welded together to form a continuous rail length.
FIG. 1B of the accompanying drawings is a graph illustrating the distribution of rail extension along the unclipped length of a rail in an ideal situation (full line), and in a practical situation (dashed line). Ideally, the extension in the rail length is evenly distributed along the unclipped length. In practice, however, friction between the rail length and the sleeper fittings causes most of the extension to occur close to the tensing point (the initially free end). The consequence of this is to concentrate the load nearest the gap and thus overstress the rail at the weld. This can lead to rail breaks. At the other end, nearest to the anchor, the rail can be unstressed and may buckle in hot weather due to thermal expansion.
In order to reduce these friction effects, existing practice is to use rollers, spaced intermittently along the unclipped length, during the tensing operation. Further, if the track is curved, additional side rollers are employed to keep the rail in the correct position and to resist the tendency for the rail to move towards the center of curvature.
In this existing practice, the rail is lifted by means of jacks, the rollers inserted under the bottom of the rail, and the jacks lowered. Rollers used in the existing practice are simple devices mounted on flat plates. Alternatively, as used in France, the rollers may be lengths of steel bar placed between the rail and the concrete sleeper top.
There are a number of problems with the above-mentioned existing practice.
The use of separate jacks, rollers and side rollers is inconvenient, and as a result the rail stressing process is time-consuming and expensive. Firstly, the use of separate pieces of equipment may necessitate the involvement of several people in order to coordinate the rail-lifting step, the placement of rollers and side rollers, and the rail-lowering step. The lifting of a heavy rail and the placing of rollers thereunder is a hazardous operation for hands that may become trapped.
Although the existing rollers relieve significant friction which would otherwise oppose extension of the rail, they still exert some drag to the free movement of the rail. The existing rollers commonly bear on the underside face of the base (or foot) of the rail. This underside face is normally close to the ground when the rail is in its working position, and is therefore subject to corrosion and may pick up debris. Rolling on this surface is not ideal.
GB 2334692 discloses a railway-rail-lifting tool having a handle at one end, and a system of jaws at the other end. In use, the jaws are placed around the head part of a section of rail, and on lifting of the handle the jaws are caused to grip on the rail for secure lifting. Such a tool requires simple lifting of the rail by hand, and as a result only very small lengths may be lifted for any reasonable length of time. Further, separate rollers and side rollers must be used in conjunction with this rail lifting tool. As above-mentioned, it is inconvenient to have to use separate rollers and side rollers.
U.S. Pat. No. 1,663,061, and GB 1035743 disclose railway-rail-lifting tools incorporating a simple lever mechanism, by which a railway rail may be lifted by hand. Separate rollers and side rollers must be used in conjunction with these rail lifting tools. As above-mentioned, it is inconvenient to have to use separate rollers and side rollers.
WO 01/96663 and FR 2488577 disclose roller clamp apparatuses for use in lifting a railway rail. The apparatuses comprise a parallel pair of spaced-apart lift roller assemblies. In the case of WO 01/96663, each roller rotates on an essentially vertical axis. In the case of FR 2488577, each roller rotates on an essentially horizontal axis.
In each case, the roller assemblies are mounted to a support for positioning the pair for clamping to a railway rail. In order to lift the rail, the apparatuses must be connected to a carrier. That is, the apparatuses must be supported by an off track machine such as a crane or gantry. It is considered disadvantageous that such an off track machine be required. Such off track machinery is commonly expensive, requires regular maintenance, may require a large electrical power source, and can be difficult to transport and position securely for use.